Modern high-power frequency converters or corresponding devices require auxiliary voltage sources that have the power of hundreds of watts in order to take care e.g. of the auxiliary voltages of control logic, gate controls of power components, as well as the power supply of adjustable cooling blowers. Usually, these auxiliary voltage sources are implemented without any large, heavy, and expensive supply-frequency converters not least because a frequency converter has to operate also during random outages by means of energy stored in its direct voltage intermediate circuit and inertia of machines supplied by the frequency converter.
A conventional auxiliary voltage source is a unit consisting of a diode bridge rectifier, a direct voltage intermediate circuit capacitor and an inverter which, at a high frequency, supplies a transformer providing a galvanic decoupling. However, the use of such an auxiliary voltage source includes some problems, such as a large current pulse which occurs upon connection to the network, and the fact that the diode bridge of the auxiliary voltage source rectifies overvoltage peaks occurring in the supplying network to its intermediate circuit.
FIG. 1 shows an example of a prior art auxiliary voltage source. Referring to FIG. 1, when an auxiliary intermediate circuit capacitor Cpa becomes charged upon being connected to the network, due to the influence of a diode bridge 100 a large current pulse is consequently generated, which has to be restricted somehow in order to prevent safety fuse blowout. A time-rate of change of voltage caused by a mechanical connecting device may easily be several thousands of volts per microsecond, in which case an auxiliary intermediate circuit capacitor Cpa having a capacitance of a few microfarads even at its minimum easily tends to cause a current peak of several thousands of amperes from a supplying rigid network (infinite bus). In the connection of FIG. 1, current is restricted by means of a charging resistance Rpa connected in series with the intermediate circuit capacitor Cpa and short-circuited by a switch Spa after the voltage of the intermediate circuit capacitor Cpa has risen sufficiently close to its nominal value. However, the charging resistance is a large, space-consuming component, and in a fault situation, when overheated, it may cause a fire or other danger.
FIG. 2 shows an example of a voltage of a network supplying a high-power network inverter as a function of time. It can be seen in FIG. 2 that the high-power network inverter causes frequently occurring overvoltage peaks 200 in the network, which are rectified by the diode bridge of the auxiliary voltage source to its intermediate circuit. It has been noticed in measurements in practice that with a network voltage of 690 volts, a difference between positive overvoltage peaks and negative overvoltage peaks may be of the order of 2000 volts, so the overvoltage peaks are absolutely to be taken into account in the dimensioning of a diode rectifier 100 of the auxiliary voltage source and the subsequent energy storage components, as well as the switching semiconductors. This, in turn, may cause high additional costs, or lead to cumbersome solutions of connecting fast components in series. In addition, if the need for auxiliary power is so small that the energy contained in the peaks is not consumed as it is received by the auxiliary intermediate circuit, the voltage of the intermediate circuit increases until the inverter part of the auxiliary intermediate circuit has to be stopped in order to prevent the switch elements from being damaged due to the increasing switching losses.